Inkjet imaging techniques have become very popular in commercial and consumer applications. Ink jet printers operate by ejecting ink onto a receiving substrate in controlled patterns of closely spaced ink droplets. By selectively regulating the pattern of ink droplets, ink jet printers can produce a wide variety of printed features, including text, graphics, images, holograms, and the like. Moreover, ink jet printers are capable of forming not just printed features on a wide variety of substrates, including not just flat films or sheets, but also three-dimensional objects as well.
Thermal ink jet printers and piezo inkjet printers are the two main types of ink jet systems in widespread use today. For both approaches, inks must meet stringent performance requirements in order for the inks to be appropriately jettable and for the resultant printed features to have the desired mechanical, chemical, visual, and durability characteristics. In particular, inks must have relatively low viscosity when jetted, yet must be able to form accurate, durable images on the desired receiving substrate. For example, a typical ink for thermal ink jetting must typically have a viscosity in the range of 3 to centipoise at 25° C., while piezo inks must typically have a viscosity in the range of 3 to 30 centipoise at the jetting temperature. The need to use low viscosity inks makes it challenging to obtain printed features with good mechanical, chemical, visual, and durability characteristics.
Solvent-based and water-based jettable inks are well known. A typical water-based ink generally comprises water, a colorant, which may be a dye and/or a pigment, one or more co-solvents, and one or more additives that are included to enhance the performance of the ink. Representative examples of such additives include one or more colorants, slip modifiers, thixotropic agents, foaming agents, antifoaming agents, flow or other rheology control agents, waxes, oils, plasticizers, binders, antioxidants, fungicides, bactericides, organic and/or inorganic filler particles, leveling agents, opacifiers, antistatic agents, dispersants, and the like.
Water-based inks have drawbacks. For industrial applications, drying is energy and equipment intensive. Drying water also takes time, and the printed material needs to be handled carefully during the relatively lengthy drying period. Water-based inks are also compatible only with a limited range of substrates, typically those on which the water is absorbed to some degree. Images formed using water-based inks typically require a protective overlaminate for outdoor applications.
Instead of water, other solvent-based inks include relatively volatile, inorganic solvents. Such inks dry more rapidly and easily than aqueous inks. However, such solvents may be toxic, flammable, or the like, requiring careful handling. These inks also tend to be compatible with only a limited range of substrates.
In order to avoid using a conventional solvent, ink compositions incorporating a free radically polymerizable diluent have been developed. The diluent not only functions as a solvent, but also functions as a viscosity reducer, as a binder when cured, and optionally as a crosslinking agent. In the uncured state, these compositions have a low viscosity and are readily jetted. However, the polymerizable monomers readily crosslink upon exposure to a suitable source of curing energy, e.g., ultraviolet light, electron beam energy, and/or the like, to form a crosslinked polymer network. Depending upon the kind of monomers incorporated into the diluent, the resultant network may provide the printed features with durability, flexibility, elasticity, gloss, hardness, chemical resistance, stiffness, combinations of these, and the like.
Conventional inks formed from radiation polymerizable monomers have some drawbacks. First, printed features formed from these materials might have a tendency to shrink when cured. Further, the adhesion, weatherability, resilience, toughness, flexibility, dot gain, and the like also may not be as good as is desired, particularly for outdoor signage applications.